Internal combustion engines, in particular diesel and spark-ignition engines, are increasingly provided with turbochargers. A turbocharger serves to compress the air supplied to the engine, whereby an increase in power can be achieved. Conversely, a predefined power output can be achieved by a turbocharged engine of smaller cubic capacity, whereby a smaller and lighter construction and more fuel-saving propulsion can be attained.
Turbochargers of this type are generally driven by the exhaust gas flow of the internal combustion engine. For this purpose the turbocharger has a turbine arranged in the exhaust gas flow. The turbocharger drives, in particular via a common shaft, a compressor which compresses the charge air of the engine.
When two turbochargers are used with an internal combustion engine, the two turbines may be arranged, in particular successively, in the exhaust gas flow of the internal combustion engine, so that one of the two turbines works in a region of relatively high pressure and it is referred to as the high pressure turbine while the other turbine works in a region of relatively low pressure and is referred to as the low pressure turbine. Likewise, the two compressors may be arranged successively in the charge air flow, so that a compressor which can be driven by the high pressure turbine works in the region of relatively high pressure and is referred to as the high pressure compressor, while the other compressor, which can be driven by the low pressure turbine, is connected upstream of the high pressure compressor and is referred to as the low pressure compressor.
By using a plurality of turbochargers with an internal combustion engine it is possible to achieve improved throttle response together with higher specific power, especially because the high pressure turbine, which is usually constructed smaller, responds more quickly. On the other hand, the low pressure turbine is used to attain maximum power. In addition, a turbocharger system with a plurality of turbochargers offers advantages with an exhaust gas recirculation system, since the high pressure turbine generates higher exhaust gas back pressure, making possible increased exhaust gas recirculation.
In order to control the turbines and therefore the charge pressure, bypass valves arranged in a branch of the exhaust system parallel to the respective turbine are provided, making it possible to bypass the turbine concerned. If a bypass valve is closed, the exhaust gas flows through the respective turbine, which is driven thereby. If the bypass valve is open, the exhaust gas flows through the branch connected in parallel to the turbine, so that the turbine is practically no longer driven.
With known turbocharger systems the bypass valves are in the form of simple valves with only two positions, by which the branches parallel to the turbines can essentially be only opened or closed. With known methods for controlling such a turbocharger system, the bypass valve of the low pressure turbine is closed at low speed or low torque of the internal combustion engine, while the high pressure turbine is controlled by opening or closing the bypass valve, thereby adjusting the charge pressure. At relatively high engine speed or relatively high torque the bypass valve of the high pressure turbine is completely open, while the charge pressure is adjusted by opening or closing the bypass valve of the low pressure turbine. In addition, a third bypass valve (compressor bypass valve) arranged in a branch of the charge air system parallel to the high pressure compressor may be provided. At low speed or low torque of the internal combustion engine, the compressor bypass valve is closed, whereas it is open at relatively high engine speed or relatively high torque.
However, an optimum mode of operation of the turbocharger system is not possible by these means. In particular, an operating mode in which the turbocharger system can be operated with optimum turbine efficiency at various working points of the internal combustion engine is not possible.
The inventors herein have recognized the above issues and provide a solution to at least partly address them. Accordingly, a method is presented for controlling a turbocharger system of an internal combustion engine. The method comprises controlling a first bypass valve of a high pressure exhaust gas turbine and a second bypass valve of a low pressure gas turbine on the basis of a turbine model, the first and second bypass valves continuously variable.
In this way, an improved operating mode of the turbocharger system, in particular an operating mode which is optimized with regard to turbine efficiency, is made possible. For example, there is provided at least one high pressure exhaust gas turbine with a first bypass valve and at least one low pressure exhaust gas turbine with a second bypass valve. In this case the low pressure exhaust gas turbine is arranged, in particular, in the exhaust gas flow downstream of the high pressure exhaust gas turbine. The bypass valves are arranged in a branch of the exhaust system parallel to the respective turbine and make it possible for the exhaust gas flow to bypass the turbine concerned. Each bypass valve may also be in the form of a valve system with a plurality of individual valves.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.